“Here we are. Four steaming milk teas, with a little dash of Epsom salt for the digestion, just like my mother used to make it!” Mr. Sadowski inched back into the room, holding a large pewter tray on which four mugs were rattling precariously. So intent was he on getting the mugs safely from tray to table that it took him a moment to realize that the four children from the museum next door had vanished, as though into thin air. Other than several newspapers that had, much to his disapproval, been disarranged, there was no sign that they had come at all.

“Well,” Mr. Sadowski said under his breath. “How very rude.” But he was not altogether sorry to be alone with his brother once more.

“Tea, Aaron?” he said, turning to the urn. He was gratified when, as always, his brother said yes.

Meanwhile, Thomas, Pippa, Sam, and Max had already returned to the museum. Luckily, they encountered no hastily assembled protest, no grumbling performers or curious journalists. They saw no one at all. Even luckier, the narrow phone booth nestled underneath the performers’ spiral staircase, which was usually occupied, most often by Quinn or Betty, was empty.

Thomas fished Ned Spode’s card from his pocket and consulted the number.

Pippa frowned. “I could have read it off for you.”

“There’s no time,” Thomas said. Before he could lift the receiver, however, the phone began to ring. Sam groaned.

“It’s probably one of Betty’s boyfriends,” he said.

“I’ll get it,” Pippa said, and snatched up the receiver.

“Pippa,” Spode said, before she’d even finished saying hello. “This is Pippa, am I right? I have some big news. Gather up the gang. We need to meet as soon as possible.”

“We have news, too,” she said, tilting the receiver away from her ear, while Sam, Max, and Thomas crowded in to listen. “We don’t think Ian Grantt really died. We think he went underground and came back—”

“As Professor Rattigan,” Spode said, cutting her off. He spoke with flinty seriousness. “I know. I’ve got it all figured out. Listen, you’re not safe where you are. Let’s talk in person. Meet at 712 West Fifty-Eighth. As soon as you can.” And he rang off so abruptly, Pippa was left staring at the receiver in her hand, a feeling of buzzing anxiety flowing all the way from her fingers into her chest.

“What did he mean, we’re not safe?” Max asked.

“What do you think he means?” Thomas answered grimly. “Rattigan’s back. And he’s coming for us.”

The address Spode had given them turned out to be on a desolate block that dead-ended at the Hudson River. The street was dominated by one hulking building, which looked, due to the narrow, boarded-up windows and the ghostly imprints of old advertising decals, to be an abandoned factory. There was no number. Across the street, a homeless man was dozing in the shadow of a doorway, his hat over his face, using a battered rucksack for a pillow.

“This must be it.” Pippa squinted up at the side of the building, scanning once again in vain for an identifying mark, or some sign of life in the windows. Nothing. But the previous block had ended with number 710, and there was nothing farther west but water, and seagulls wheeling through the air.

Max shifted from foot to foot, occasionally palming at her pockets, where Pippa could see she’d placed three separate blades in different sizes.

“Should we knock?” Sam asked. Thomas shrugged and Sam stepped forward, raising a fist.

Max intercepted him. “Don’t you do it,” she said. “You’ll knock the whole bugging door down.” She knocked three times on the door. Pippa could hear the sound echoing hollowly within. After a minute, the door opened, seemingly on its own, revealing a triangle of murky darkness.

Sam stepped forward—to impress Max, Pippa was sure, since his fingers were trembling ever so slightly. She fell into step behind him, and Thomas and Max took up the rear.

“Hello?” Sam called out as they passed inside.

The door slammed shut behind them and Pippa’s heart leaped into her throat. She spun around. But no one was there.

“This way!” Spode’s voice sounded, very faintly, from somewhere deeper within the building. Pippa felt instantly better. So they were not alone in a creepy building after all.

They moved cautiously, as a group, further into the murky darkness, skirting metal gurneys, dustbins, and coiled cables, and ducking under the chutes that crisscrossed the low-hanging ceiling. The dusty floor was pitted with railway tracks, winding like iron snakes from one end of the factory to another. An old trolley car, only half painted, loomed motionless in the dark. As they skirted around it, Pippa had a sudden fear that it would come blazingly to life, horn blaring, windows flashing with light, and run them down.

There was still no sign of Spode.

“Where are you?” Thomas shouted.

“Just a little further!” Spode called back, although he sounded just as distant as before. Was he moving? Surely, he wouldn’t be. But Pippa’s arms were prickling with gooseflesh. It was cold in the factory, as if they’d passed into a different world, a different season.

Up ahead, Pippa detected a flickering light, as if someone were pacing back and forth with an old-fashioned lantern. Spode. As they drew closer, she could make out the planes of his face, lit up harshly in the gaslight, all ridges and angles. He looked, Pippa thought, like a stranger.

Unconsciously, she began to move faster. Sam had fallen behind. His attempts at bravery had obviously failed him, and he was sticking close to Max.

“There you are,” Pippa said, and even she could hear how thin and tremulous her voice sounded. “What are you doing all the way back—?”

She didn’t finish her sentence. Several things happened at once. Spode turned to her and there was a sudden, blinding explosion of light, a flare so bright Pippa shut her eyes instinctively. She couldn’t see. Her vision was obscured by floating dots of color, and for one terrifying second she believed she’d really gone blind. Everyone was shouting, and someone—it felt like Sam—knocked into her from behind. She went sprawling, hitting the ground hard and biting down on her cheek, tasting blood.

And in that moment of impact, it was as if she was jolted out of her body, like her mind had received a big punt kick. And suddenly she was Pippa but also Max, or Pippa-as-Max, crouching in a corner of Max’s mind and feeling her fear and her terror and her desperate, fumbling desire for her knives. . . .

But Pippa-as-Max knew that it was too late. Someone tackled Max from behind, wrestling off her jacket and removing the knives in her pockets. Pippa-as-Max could smell him, the distinctive combination of chemical and sulfur, of something pickled and left too long on a shelf. . . .

And as Max cried out and Pippa was jolted once again into her own body, she knew: they had walked right into a trap.

“Well, well, well.” Gradually, a steady light began to penetrate through the blur of Pippa’s vision. Another gas lantern flared to life—and standing next to it, his long, pale fingers curled around the handle, was a smiling Rattigan. “So very nice of you to join us.”